1. Field of the Invention
The invention relates to a balanced-unbalanced radio frequency transformer (balun) and its use. Its field of application is that of processing radio frequency signals and more particularly that of mobile telephony. The object of the invention is to reduce the size of a transformer of this kind implemented in the form of lines printed on a main card in order to reduce the unit cost.
2. Description of the Prior Art
These transformers have fixed dimensions because of their nature. They are seen as reciprocal passive octopoles (four ports). Each port is connected to a respective output by a line. One port is connected to ground and a radio frequency signal of amplitude A is applied to a second port. Two output signals are obtained, each on one output, having an amplitude A/k and a relative phase of xcex8xc2x0. The factors k and xcex8 are respectively determined by the distance between the lines constituting the transformer and by the length and the width of each line. Reciprocally, two signals of amplitude A/k and with a relative phase of xcex8xc2x0 are applied at the output to recover a signal of amplitude 2A/k at the second port. The physical dimensions of the components constituting the transformer are imposed by the relative phase xcex8. A transformer of this kind therefore has minimum overall dimensions.
At the present time radio frequency transformers include an insulative material substrate plate. Each of the lines constituting the transformer is placed on one face of the substrate plate. With the aim of economizing on area the lines follow parallel paths which are either a straight line or a loop. The lines are therefore on two levels, on respective opposite sides of the substrate. These transformers are also sometimes implemented in coaxial cable. The two coupled lines are then the core and the shield of the coaxial cable. The problem of area congestion is then associated with a problem of volume congestion.
A mobile telephone may require several radio frequency transformers. If they are implemented in coaxial cable, for example, they take up room in terms of thickness and area, which can be prejudicial to the design and to the cost of the mobile telephone. If other solutions are adopted the thickness problems can be solved but the area used by the transformers cannot be used for other functions and the size of the mobile telephone will be increased.
The invention solves the above problem by providing a radio frequency transformer implemented on at least four levels. The four levels are layers of metallization of a multilayer printed circuit, for example. In one example the printed circuit has six layers. Implementing the transformer on four levels is not a problem with regard to the printed circuits used, since most mobile telephones already use printed circuits with six-level technologies. The area congestion of the transformer of the invention on printed circuits is then greatly reduced. Using the inner layers of a multilayer circuit frees up the surface on which surface-mount components can be mounted.
The invention therefore provides a radio frequency transformer including two main lines which have parallel routes and which lie on at least four different levels materialized by four parallel planes, two of which levels correspond to a first of the two main lines and two other of which levels correspond to the second of the two main lines, and four ports consisting of the ends of the main lines.
The invention also encompasses the use of a transformer of the above kind in a mobile telephone modulator or demodulator.
The invention will be understood better after reading the following description and examining the accompanying drawings. The drawings are provided by way of non-limiting example of the invention.